Hi,
treadmill motors rely on current to produce torque. Provided you have a big enough driver and power supply the treadmill motor will
keep producing more torque until it overheats and blows up.
I think your problem is that the motor is rated at 4200 rpm so it will have modest torque.
Power= torque x rotational speed
I calculate in metric units so I will stick with them....if your a Yank....too damn bad!
1.75 hp = 1330W (hp x 760)
4200 rpm = 439.8 radians/sec (2 x PI x rpm/60)
torque =1330/439.8
=3.02Nm
3Nm is pretty modest for a spindle. If you use a gear or belt reduction the torque increases but speed decreases while power stays about the same.
I suspect that you are not using the power that your motor has very well.
Craig